Micro fluidic systems in which a semi-permeable membrane is attached to one or more tubes are known per se. Such systems are, e.g., used in dialysis probes. Previously the membrane has been attached to the tube(s) by means of welding, melting or gluing the membrane to the tube(s). This is disadvantageous because it is relatively difficult, and thereby relatively expensive, to produce the micro fluidic systems in this manner.